It is known that many materials, such as carbon fiber/carbon matrix, carbon fiber/silicon carbide matrix, and silicon carbide fiber/silicon carbide matrix composites in which the silicon carbide fibers are coated with carbon; carbon fibers; graphite; and certain metals have properties which make them attractive for use in aerospace and other applications in which their susceptibility to oxidative deterioration at elevated temperatures is a serious disadvantage.
As disclosed in European Patent Application 0 325 483 (Niebylski), such normally oxidizable materials can be protected from this oxidative deterioration by providing them with ceramic coatings derived from organoborosilazane polymers; and Niebylski's copending U.S. application Ser. Nos. 414,262, 414,464, and 414,768, all filed Sep. 29, 1989, show that the degree of protection provided by the coatings can be increased by dispersing certain metal borides, e.g., silicon borides, in the organoborosilazane polymer compositions before coating them onto the normally oxidizable materials and pyrolyzing them.
Niebylski's copending application Ser. Nos. 462,791, filed Jan. 10, 1990, and 466,482, filed Jan. 17, 1990, teach that even greater protection can be achieved when silicon metal particles, and optionally also aluminum-silicon eutectic and/or silicon carbide particles, are dispersed in the organoborosilazane polymer compositions together with the silicon boride particles; and the latter of these two applications further discloses that decreased moisture sensitivity can be attained by also dispersing a Group IIa metal salt in the compositions.
The utility of the aforementioned coating compositions in forming one or more of the strata of multilayer ceramic coatings on normally oxidizable substrates is taught in copending application Ser. Nos. 446,184 (Niebylski), filed Dec. 5, 1989, and 466,225 (Niebylski et al.) and 466,594 (Niebylski), both filed Jan. 17, 1990. In the former two of these applications, it is also taught that, although cracking is apt to occur when ceramic layers derived from polysilazane compositions are applied over ceramic layers derived from organoborosilazane polymer compositions (such as those described in the preceding paragraphs), the cracking tendency can be minimized by the use between those different layers of a buffer layer derived from a composition comprising as the essential components a Group IIIb metal hydrocarbyloxide, a Group IVa metal hydrocarbyloxide, a (dialkylamino)metal of Group IVa, and a polysilazane and/or (dialkylamino)silane.